Difference in exposure of water birds to covered and uncovered float muskrat sets

Muskrats Ondatra zibethicus are a popular furbearer species across much of North America. Float sets have gained popularity due to the ease of use and effectiveness of capturing muskrats. Little to no research has been conducted on muskrat float sets, especially on the impacts the float sets have on non-target animals. In North Dakota, USA, regulations allowed trappers to use float sets during the spring season, but float sets were required to have a covering made of wire mesh, wood or plastic and no opening larger than 20.32 cm (8 inches) in an effort to minimize the incidental take of non-target species. We aimed to determine if there was any non-target capture injury or mortality risk on float muskrat sets. We conducted a study to compare rates of incidental take in covered (2.54 x 3 2.54 cm and 15.24 x 3 15.24 cm wire mesh) and uncovered float sets. We trapped muskrats in fall (1191 trap nights) and spring (3054 trap nights) from 2012-2014 at four study areas in North Dakota. Over four trapping periods (two fall and two spring seasons), 490 muskrats and seven non-target species were captured. Non-target species included three black-crowned night heron Nycticorax nycticorax, two blue-winged teal Anas discors and two painted turtles Chrysemys picta. All avian non-target species were captured on uncovered floats. Camera trap data showed that ducks were 10.1 times less likely to be on floats than other types of water birds (e.g. herons). Covers did not negatively influence muskrat captures, but smaller mesh sizes appeared to deter birds from climbing on top of floats. All but one avian non-target capture occurred after 1 May (closing of North Dakota's spring muskrat trapping season) each year, suggesting that season dates may be an important factor to consider in attempts to reduce incidental take of protected bird species.